Name the type of reaction for the following:
$(i)$ Silver items turning black when left exposed to air.
$(ii)$ Change in colour of copper sulphate from blue to green on dipping an iron nail in it.

Which of the following gases is combustible?

Translate the following statements into chemical equations and then balance them.
$(a)$ Hydrogen gas combines with nitrogen to form ammonia.
$(b)$ Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
$(c)$ Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
$(d)$ Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Which one of the following is a chemical change? Give reason also.
$A$. Burning of wax
$B$. Melting of wax

Write the balanced chemical equations for the following reactions and identify the type of reaction in each case.
$(a)$ Thermit reaction,iron $(III)$ oxide reacts with aluminium and gives molten iron and aluminium oxide.
$(b)$ Magnesium ribbon is burnt in an atmosphere of nitrogen gas to form solid magnesium nitride.

Write the balanced chemical equations for the following reactions and identify the type of reaction in each case.
$(a)$ Nitrogen gas is treated with hydrogen gas in the presence of a catalyst at $773 \ K$ to form ammonia gas.
$(b)$ Sodium hydroxide solution is treated with acetic acid to form sodium acetate and water.